This invention relates to a single-step polymerisation process for producing impact-resistant, thermoplastically processable moulding compositions by the radical-induced polymerisation of aromatic monoalkenyl compounds, ethylenically unsaturated nitriles, and optionally of other copolymerisable compounds, in the presence of soluble, gel-free polybutadiene rubber as a graft base in the solvent.
The process according to the invention is characterised by its simple polymerisation technique and cost-effectiveness, and the products according to the invention are characterised by their high impact-resistance, and at the same time by their high hardness, translucency, matt surface, light self-colour and outstanding processability.
Low polymerisation temperatures and pressures impart a high degree of operational reliability to the process according to the invention.
The object of the invention is an economical, operationally reliable process for producing thermoplastically processable ABS moulding compositions, which can be carried out in equipment which is technically simple and which provides ABS with good processing and application properties and with outstanding impact-resistance of the graft rubber. A further object of the invention is to provide ABS moulding compositions having a high rubber content which are suitable as mixture components and impact-resistance modifiers, and which are equivalent to the moulding compositions which can be produced by aqueous emulsion polymerisation.
The present invention relates to a process for producing impact-resistant thermo-plastic moulding compositions, which is characterised in that a monomer mixture comprising
90 to 20 parts by weight of an aromatic monoalkenyl compound,
10 to 50 parts by weight of an ethylenically unsaturated nitrile,
0 to 30 parts by weight of other copolymerisable compounds
is polymerised by a radical mechanism in the presence of
15 to 50 parts by weight, per 100 parts by weight of monomers, of a soluble, gel-free butadiene polymer and in the presence of
50 to 200 parts by weight, per 100 parts by weight of monomers, of a solvent, (monomer mixture+butadiene polymer+solvent)
wherein the solvent is selected from the group comprising an aliphatic (C1-C8) or cycloaliphatic alcohol, ketone, ether, ester, nitrile (A) or mixtures thereof or a mixture of (A) with an aliphatic (C4-C10), cycloaliphatic or aromatic hydrocarbon (B) in an A:B weight ratio of 100:0 to 30:70, and the polymerisation is conducted as far as a polymer content of the total mixture of 30 to 70% by weight, with thorough mixing and optionally with the subsequent addition of a regulator and initiator, so that the isolated thermoplastic moulding composition contains 20 to 40% by weight of butadiene polymer.
In a preferred embodiment, the polymer content of the total mixture is 30 to 60% by weight, particularly 35 to 50% by weight, the total content of solvent is 25 to 60% by weight, and the residue comprises unconverted monomers, these features being present simultaneously. The content of butadiene polymer in the moulding composition is 20-40% by weight, preferably 25 to 40% by weight, particularly 28 to 40% by weight.
The products according to the invention are characterised by a low degree of grafting between 0.05 and 1, preferably between 0.2 and 0.8. In this connection, the degree of grafting is the ratio             gel      ⁢              xe2x80x83            ⁢      content        -          rubber      ⁢              xe2x80x83            ⁢      content            rubber    ⁢          xe2x80x83        ⁢    content  
The gel content means the fraction of the moulding composition which is insoluble in tetrahydrofuran. The products contain a finely dispersed rubber phase with particle sizes between 0.05 and 10 xcexcm, preferably between 0.05 and 5 xcexcm.
In this respect, the particle size denotes the weight average of the particle size distribution. It is determined using an ultracentrifuge.
The products with a high rubber content which are produced according to the invention can be used as such; however they are preferably mixed, by the normal methods of plastics processing, with co- or terpolymers of aromatic monoalkenyl monomers, ethylenically unsaturated nitrile monomers and optionally with other copolymerisable vinyl monomers and/or maleic monomers, in a weight ratio of 1:0.25 to 1:10, to form moulding compositions which likewise have a high impact-resistance but the hardness of which is improved.
Bulk and solution polymerisation for the production of rubber-modified moulding compositions are known, and are described in Houben-Weyl, Methoden der Organischen Chemie, Volume E 20/Part 1, pages 182-217, Georg Thieme Verlag, Stuttgart.
Small proportions of solvents of groups (A) or (B) can be added during bulk polymerisation (see US-PS 4 587 294, US-PS 5 286 792, EP-A 376 232, US-PS 5 278 253). However, high temperatures are required in the final stage, the polymerisation has to be conducted under elevated pressure, and high viscosities occur which can only be dealt with in a controlled manner by the use of expensive techniques such as tower reactors, static mixer reactors, paddle reactors, amongst others.
The use of solvents (B) on their own has also been described for bulk polymerisation (see EP-A 277 687, EP-A 657 459, US-PS 3 538 190, DE-A 2 516 834, US-PS 3 449 471), where they are used in small amounts for dilution purposes.
The contents of rubber in the polymerisation mixture are likewise low.
Higher contents of rubber are not possible, due to the extremely high viscosities which then occur.
Amongst their other features, non-aqueous polymerisation processes result in improved product properties, such as self-colour, translucency, rubber efficacy and matt surfaces. Aqueous effluents do not arise. These advantages are achieved according to the present invention by a simple technique, at low polymerisation temperatures of 60 to about 150xc2x0 C., preferably 70 to 120xc2x0 C., under normal pressure or under only slightly elevated pressure, wherein the viscosities which occur are less than 150 Paxc2x7s only. ABS resins having a high rubber content of up to about 40% by weight can also be produced according to the invention. ABS resins with a high rubber content such as these are required as additives for thermoplastics. Hitherto, it has only been possible to produce them by polymerisation in aqueous emulsion. The products according to the invention are characterised by their outstanding processing properties and properties in use. Due to the rubber content in the ABS resin being considerably higher than that obtained with bulk polymerisation, and due to the simpler technique employed, the economic disadvantages of the larger amounts of solvents used are also compensated for.
The essence of the present invention is the conjoint use of larger amounts of a solvent or of a solvent mixture of group (A) defined above, together with high rubber contents in the monomer mixture and in the final product and incomplete conversion of the monomers. When solvents or solvent mixtures of group (A) and optionally of group (B) are used in the given weight ratios of 1:0 to 3:7, it is possible rapidly to pass through a phase inversion when sufficient conversion has occurred, despite the higher rubber content, so that a finely dispersed phase of graft rubber is formed. On departing from the ratios according to the invention, e.g. when exclusively using solvents of group (B), this phase inversion is no longer obtained and a sharp increase in viscosity occurs, or a premature phase inversion occurs, whereupon the disperse phase which is then formed is of a coarse particulate nature due to insufficient grafting, so that poor product properties are obtained and thick deposits occur in the reactor. Although the solvents of group (A) do not dissolve rubbers, they can be added in a large amount, even before the commencement of polymerisation and even when there are high contents of rubber in the monomer mixture, without precipitation of the rubber occurring. On the contrary, particularly advantageous Theological behaviour of the polymerisation solutions and favourable product properties can be obtained by this measure.